1. Field of the Invention
The present invention relates to a method for manufacturing a solid-state image capturing apparatus, and to an electronic information device; and particularly to a method for forming a photoresist mask in a method for manufacturing various solid-state image capturing apparatuses, such as a CCD type image sensor and a CMOS type image sensor, and an electronic information device having therein the solid-state image capturing apparatus obtained by the method for manufacturing a solid-state image capturing apparatus.
2. Description of the Related Art
A conventional solid-state image capturing apparatus, such as a CCD image sensor, generally includes a pixel array constituted of a plurality of pixels, the pixel array being formed on a surface section of a semiconductor substrate; and a peripheral circuit positioned in a peripheral area of the pixel array for reading out a pixel signal from each pixel that constitutes the pixel array. In addition, a portion other than a photodiode section, constituting a pixel on the surface section of the substrate, is shielded by a light shielding film.
FIG. 8 is a diagram illustrating such a conventional CCD image sensor, and FIG. 8(a) schematically illustrates the configuration of the CCD image sensor.
A CCD image sensor 50 includes: a plurality of photodiode sections (photoelectric conversion sections) 30a arranged in a matrix (two dimensionally) for performing a photoelectric conversion; a vertical transfer section (vertical CCD) 30b provided by corresponding to each column of the photodiode section for transferring a signal charge generated by each photodiode section 30a in a vertical direction; a horizontal transfer section (horizontal CCD) 30c for transferring a signal charge from the vertical transfer section 30b in a horizontal direction; and an output section 30d connected to a terminal of the horizontal transfer section 30c for amplifying the horizontally transferred signal charge to output it as a signal voltage. Herein, each photodiode section 30a constitutes each pixel Px in the CCD image sensor 50, together with a corresponding part of the vertical transfer section 30b, and the output section 30d is constituted of a MOS transistor.
FIG. 8(b) is a plan view illustrating the layout of the photodiode 30a and the vertical transfer section 30b that constitute the CCD image sensor 50. FIG. 9 is a diagram illustrating a cross sectional structure along the line V-V in FIG. 8(b).
For example, as illustrated in FIG. 8(b) and FIG. 9, a P-type semiconductor well area 32 is formed in a surface area of a semiconductor substrate 31, such as an N-type silicon substrate; and an N-type impurity diffusion area 33 constituting the photodiode section (photoelectric conversion section) 30a is arranged in a matrix in the P-type semiconductor well area 32. Herein, a P-type (P++) impurity diffusion area 34 is formed on the surface of the N-type impurity diffusion area 33; and the photodiode section 30a has a buried photodiode structure.
Further, in the P-type semiconductor well area 32, an N-type impurity diffusion area 37 and a P-type (P+) impurity diffusion area 36 are formed along the column of the photodiode sections 30a that form a line in a vertical direction. In addition, a P-type impurity diffusion area 35, which constitutes a readout gate area functioning as an electric charge readout section, is positioned between the photodiode sections 30a and the vertical CCD section 30b that is positioned on one side thereof. Further, P-type (P+) impurity diffusion areas 38 are formed between adjacent photodiode sections 30a and between the photodiode section 30a and the vertical CCD section 30b, the P-type (P+) impurity diffusion areas 38 functioning as a channel stop area for electrically separating the photodiode sections 30a and the vertical CCD section 30b. 
Further, as illustrated in FIG. 9, an insulation film 39, such as a SiO2 film, is formed on a surface of the P-type semiconductor well area 32, and an electric charge transfer electrode 41 is formed above the insulation film 39 with an insulation film 40, such as a Si3N4 film, interposed therebetween in the N-type impurity diffusion area 37 that constitutes the vertical CCD section 30b. The electric charge transfer electrode 41 is covered by an interlayer insulation film 42. In addition, a light shielding film 43 is formed on the interlayer insulation film 42 such that the light shielding film 43 is configured to include an opening at a portion corresponding to the high concentration P-type impurity diffusion area 34 to allow light to enter only at the photodiode section 30a. This light shielding film is formed of a metal, such as aluminum (Al), tungsten (W), and titanium (Ti), and the light shielding film is formed by selective etching for the metal film.
Further, an interlayer insulation film 44 and further a BPSG film 45 as a planarization film, the BPSG film 45 being a surface protection film constituted of PSG (phosphor silicate glass), are consecutively formed on the entire surface of the P-type semiconductor well area 32. Normally, a microlens (not shown) and a color filter (not shown) are formed on such a planarization film in a solid-state image capturing apparatus.
Conventionally, in a solid-state image capturing apparatus of this type, a method is taken where a photodiode section (photoelectric conversion section), for example, is formed also in a deeper area of a substrate in order to improve the sensitivity characteristic as well as to maintain and improve the sensitivity characteristic against reducing the pixel size, so that a signal charge accumulating section is enlarged in the photodiode to increase an effective area of the signal charge accumulating section (see Reference 1, for example).
Thus, in order to form the photodiode section (photoelectric conversion section) to reach a deeper area in the substrate for improving the sensitivity, it is necessary to increase the energy amount of an impurity implantation. Therefore, when an impurity implantation preventing film is formed with a photoresist on the substrate in order to form a desired impurity implantation area (i.e., the above mentioned impurity diffusion area) that reaches the deeper area in the substrate, a sufficient thickness of the photoresist is required so as not to allow the impurity to reach the portion on the substrate where the impurity implantation should be avoided.
On the other hand, the surface area of the photodiode section (photoelectric conversion section) per pixel is being reduced along with an increase in the number of effective pixels as well as the reduction of the chip size of latest solid-state image capturing element devices. Due to this, regarding an exposure light source for processing a photoresist film used for the impurity implantation preventing film for forming the photodiode section (photoelectric conversion section), it is not possible to deal with the miniaturization of the processing size for a photoresist film thickness without using a light source of a KrF excimer laser (λ=248 nm), instead of a conventional i-line (λ=365 nm).
In an exposure apparatus using a KrF excimer laser as its light source, the optical members are limited to those with high transmissivity up to and including the wavelength of 248 nm, and optical systems using only synthetic quartz are common. Therefore, it becomes necessary to use a laser in which the oscillation wavelength is narrowed (up to 0.3 pm), the exposure intensity by the layer being weaker compared to an exposure apparatus of i-line and the like, since a single optical member cannot correct a color aberration, and as a result, it is necessary to use a resist which has a high sensitivity.
Accordingly, as the method for achieving a high sensitivity, an explanation will be described with reference to a positive type high sensitivity resist.
A resist film constituted of a high sensitivity resist generates acid from a photo-acid-generating agent combined in the resist film at exposure of the resist film. Then, the acid acts, for a large number of times, on a base resin, in which a dissolution inhibition group that is desorbed by acid is introduced, thereby making the dissolution inhibitor group desorbed from the base resin of the exposure portion of the resist to increase dissolution of the base resin to a developing solution. A large number of reactions that cause the change in the dissolution are caused by acid in the high sensitivity resist film having such a structure, even when the quantum yield of acid to be generated by exposure is 1 or less. As a result, the quantum yield of acid in an effective reaction becomes far greater than 1. Therefore, such a high sensitivity resist is usually referred to as chemical amplification type (system) resists.
Types of photo-acid-generating agents, which are combined with a photoresist for a KrF excimer laser light source, the photoresist being one of the chemical amplification type resists, included are onium salt including iodonium salt and sulfonium salt, halogen compound, ortho-nitrobenzyl ester, alkylsulfonate acid ester, and sulfonyl compound.
FIG. 10 illustrates iodonium salt O1 (FIG. 10(a)) and sulfonium salt O2 (FIG. 10(b)), as onium salt which is a representative photo-acid-generating agent. FIG. 1 illustrates a halogen compound H1 (FIG. 11(a)) and a halogen compound H2 (FIG. 11(b)), as a photo-acid-generating agent. Further, FIG. 12 illustrates sulfonate acid ester S1 to S10 (FIG. 12(a) to (j)) as photo-acid-generating agents.
When a minute processing size for a resist film having a large thickness is achieved as described above while the verticality of the resist taper angle is maintained at an opening portion of the resist, iodonium salt is used as onium salt due to the controllability of the diffusion distance of the acid generated and the like.
Reference 1: Japanese Laid-Open Publication No. 2005-332925